Biochar stoves could fight climate change

Stoves convert biomass, such as garden waste, into charcoal, which can be a carbon store

At the moment there is no mass-market consumer model

Some say the technology is still unproven

(CNN) -- Good news for everyone who loves a barbecue: soon you could be helping save the world as you flip your burgers.

A new generation of small, barbecue-style stoves could soon be making it possible to sequester carbon while you cook -- with the added advantage of producing fertilizer for your garden in the process.

The stoves are fueled by biomass -- which can be almost any garden waste, from cuttings to wood shavings. Using a process called pyrolysis the biomass is heated with little or no oxygen, to produce charcoal (known as biochar).

If you bury the biochar in the garden it functions as a long-term carbon store that could improve your soil -- with studies at the University of Bayreuth, Germany, showing that biochar may nearly double plant growth in poor soils.

Of course, the impact of biochar stoves would remain fairly insignificant globally if we were relying on the odd suburban summer cook-off, however hungry the guests were.

But around the world three billion people still cook and heat their homes with stoves or open fires, burning wood, straw, dung, or coal, according to the World Health Organization.

These inefficient technologies not only contribute to global warming, they can harm respiratory and cardiac health. The WHO says exposure to toxic smoke from traditional stoves and fires accounts for nearly two million premature deaths annually.

The first is known as a Top-Lit Updraft Gasifier, which works by placing biomass in a cylinder that is fed air from underneath.

When the biomass is lit, the flame seals the cylinder at the top and the biomass begins to pyrolyze in the cylinder below. By the time cooking is complete, the biomass has become biochar.

The second type is the Anila-type stove, which consists of two concentric cylinders. The outer cylinder is covered, while the inner one is open.

Most of the biomass is placed into the outer cylinder, with a smaller amount in the inner cylinder. The biomass is then lit in the inner cylinder, and the flame heats and pyrolyzes the biomass in the outer cylinder.

But as yet there is no mass-market consumer model. That's one reason why some are reserving judgment as to how useful these stoves can actually be.

Almuth Ernsting, of campaign group Biofuelwatch, told CNN: "On balance of course it is important that people have access to safe, efficient stoves instead of cooking over inefficient, dangerous open fires.

"But we haven't seen anything that shows that these biochar stoves are at a technical stage where they can be rolled out."

But biochar has potential that extends beyond being the byproduct of household cooking stoves.

Environmentalist and author James Lovelock says, if adopted on an industrial scale, with farmers disposing of crop waste to produce and bury biochar, the potential of biochar is so great he sees it as our only hope if we are to avoid catastrophic climate change.

Writing in the Proceedings of the National Academy of Sciences, Nobel Laureate Mario Molina and others argue that the carbon we have emitted into the atmosphere will be with us for generations -- unless we remove it through sequestration technologies, such as biochar.

Of course, any new technology carries with it the risk of hype, and at this stage research is still ongoing to establish exactly how potent it can be.

Ernsting, for one, has doubts about the effectiveness of biochar as a fertilizer. "When it comes to soil fertility ... it is clear it is a very complex picture and there are virtually no long-term field trials," she said.

"What happens will very much depend on what type of soil you are dealing with. No-one can yet say 'biochar will do this.' We just don't know -- there will be huge variety.

"Given this uncertainty, what worries us is the idea that [biochar] is being recommended to farmers in Africa and Asia."

While the research continues, the advocates maintain that barbecuing with biochar stoves could be a simple way to fight climate change.

"Consumers in the west should be demanding these stoves be stocked at their local stores," said Zaelke.

"This is something we can all be doing. It's easy -- and yet the impact could be truly dramatic."